1. Field of the Invention
The present invention relates to a vibration isolation structure of a power unit in which an internal combustion engine is integrally provided with a transmission. More specifically the present invention relates to such a vibration isolation structure involving torque dampers of the power unit.
2. Background Art
In a conventional power unit, plural powertrain mechanisms intervene in the process of transmission of rotational power of a crankshaft of an internal combustion engine to an output shaft via a clutch system and a transmission. In general, in the clutch system, a torque damper that buffers torque fluctuation accompanying switching of the clutch is provided for a primary driven gear.
Furthermore, in Japanese Patent Laid-Open No. 2003-193854 (JP 2003-193854) there is disclosed an example including plural torque dampers that buffer torque fluctuation accompanying gearshift of a transmission to intend to suppress the vibration of a power unit as much as possible.
In a structure of (JP 2003-193854), a cam-type damper is provided between the input side of a constant-mesh gear transmission and a clutch output shaft and a torque damper is provided between a countershaft as the output of the transmission and an output shaft of the power unit.
In the conventional power unit including the transmission, large torque fluctuation is caused depending on the timing at which the present shift stage is switched at the time of gearshift and the driving reaction force from the road surface. To buffer this large torque fluctuation, in the structure of (JP 2003-193854), the torque damper is provided on both the input side and output side of the transmission with the intention of suppressing the vibration of the power unit as much as possible.
However, providing the torque damper on each of the input side and output side of the transmission increases the size of the power unit. In addition, when large torque fluctuation is caused in the transmission, a pair of transmission gears of the shift stage to which a driving force is not transmitted act as inertia mass directly. Thus, it is not necessarily easy to suppress the vibration due to the torque fluctuation even when the plural torque dampers are provided.